This invention relates to polymer modified cementitious compositions; and more particularly to polymer modified cementitious compositions used as stucco and similar coatings.
The use of cementitious composition to provide durable wall surfaces are well known. One common example is stucco, which may provide a durable and/or decorative coating over various substrates. One application to which the present invention is particularly well suited is a hard, durable finish applied to exterior insulation.
Thin layers of cementitious compositions, such as stucco, are prone to cracking and dehydration. Thus various additives have been used to modify the basic cementitious compositions characteristics. Many building contractors have used polymers to modify stucco compositions. Typically a polymer-water emulsion is admixed with cementitious composition at the job site. While an improvement over non-polymer modified cementitious compositions, this procedure has several disadvantages.
First, there is a tendency for contractors to use less than the recommended polymer levels to reduce expense, since the polymers are generally quite expensive compared to other cementitious composition ingredients. When the polymer comes in a liquid suspension, it is easy for the contractor to just add more water and less liquid polymer when mixing the ingredients of the stucco.
Another drawback to liquid polymers is the freight costs for shipping the weight and volume added by the water. Additionally, such water-polymer emulsions must be protected from freezing. Perhaps the most serious drawback is that the polymers in such emulsions tend to settle out over time. Thus, depending on the time since its manufacture, the polymer concentration will vary over the heighth of the container in which it is stored. A contractor, if not careful to remix the emulsion, may pour off from the top of a container an insufficient amount (or if from the bottom of the container, an excess amount) of polymer to admix with the cementitious composition, even though the recommended volume of polymer emulsion is used.
In addition to the drawbacks of using a water-polymer emulsion, the polymer itself may result in undesirable side effects. In humid conditions, a stucco containing a reemulsifiable polymer will tend to soften as moisture in the air wets the polymer in the stucco, even after the stucco ages the required amount of time to cure. This makes the stucco less durable, and may also lead to mildew formation.
There are other aspects of common stucco compositions where additives, while improving the basic cementitious composition characteristics, also present difficulties. Fiberglass fibers may be used to improve the strength characteristics of the cementitious composition. However, because of the alkaline nature of cement, the strength of these fibers are destroyed over time. Thus alkaline-resistant coatings have been applied to fibers used for this purpose. The result has not always been successful as the coating may be scrubbed off as the fibers are mixed into the aggregatecontaining cementitious composition.
The cost of a stucco wall is heavily influenced by the cost of labor to apply the cementitious coating to the wall. It is very desirable to have a cementitious composition which can be pumped, allowing faster application and thus reduced labor costs. However, as a practical matter, it has not heretofore been possible to pump common base coat compositions containing fibers, partly because the fibers, especially fiberglass, tend to bridge across the passageways and orifices of the pumping appartus, plugging it up.
Another difficulty in producing a desirable cementitious composition is finding compatable additives so that when combined, the additives provide the desired characteristic without producing adverse influences on other additives. For example, some additives, such as wetting agents, which may be used to make the cementitious composition more fluid, may not be compatible with polymers used to improve the cured characteristics of the composition.
Finally, the size distribution of aggregate used in the cementitious composition may often produce inferior cementitious compositions, especially in thin layer applications.
Producing a cementitious composition with a combination of superior strength characteristics and ease of application requires the simultaneous solution to these and other problems.